Japanese Laid-Open Patent Publication No. 2001-349110 and Japanese Laid-Open Patent Publication No. 2001-349117 exist as known examples of electronic key systems for a vehicle. The electronic key systems for a vehicle disclosed in these patent documents have activation means (switches) arranged in a door handle and trunk lid of the vehicle. If a user operates (activates) these activation means, communication with the electronic key commences, comparison is carried out between an ID transmitted from the electronic key and an ID registered in the controller, and a door lock is released at a stage when a result is obtained that there is ID matching.
Also, in the patent documents mentioned above, an activation means (a switch) is also provided in an ignition knob, and after a user has boarded a four-wheeled vehicle, communication with the electronic key is carried out again, by operation of the ignition knob, and an ID comparison is carried out in order to allow the engine to be started. Then, when results of the ID matching comparison have been obtained, the engine is started.
That is, with the electronic key system for a four-wheeled vehicle of the related art, the electronic key has functions for unlocking a door and allowing starting of the engine.
An electronic key system that has a transmission antenna provided for each door of a four-wheeled vehicle, which can release only the door that a portable device is closest to independently of the other doors of the vehicle, has also been proposed as related art (see, for example, Japanese Laid-Open Patent Publication No. 10-317754).
Also, as an electronic key system applied to a motorcycle, a system has been proposed that is intended to achieve reliable theft prevention, by generating a random number based on a time taken from a power supply of an immobilizer turning ON to starting of the engine (start time data), generating an authentication key and an authentication code by encoding information inherent to the key based on the random number, and carrying out authentication using the authentication key and the authentication code (see, for example, Japanese Laid-Open Patent Publication No. 2001-12123).
Since the electronic key is carried by the user, there is a danger of the key being dropped inside the vehicle when the key is not in use. In the case of a four-wheeled vehicle, however, there is only a very low possibility of the electronic key being lost after use, even if it is dropped inside the vehicle.
However, when the electronic key system for a four-wheeled vehicle is applied, as is, to a motorcycle, for example, if the electronic key is dropped by the user after starting the engine, there is not a problem if it is noticed that the key has been dropped. However, if the motorcycle moves without noticing that the key has been dropped, the key may become lost and it will not be possible to restart the engine.
In the related art, in a system that has been applied to a motorcycle, theft prevention is a main consideration, and for example, after boarding the vehicle once authentication of the normal user is complete, the security system is stopped. As a result, if the electronic key is dropped during travel, for instance, there is a danger that it will not be detected.
Naturally, even with a four-wheeled vehicle, if the electronic key is dropped close to the door, the probability of the electronic key becoming lost is high, and in this case also, it will not be possible to restart the engine.
The present invention has been conceived in view of these types of problems, and an object of the present invention is to provide an electronic key system for a vehicle that can output a warning if a user drops an electronic key while the vehicle is stopped or during travel, and that can reduce, to as low as possible, the probability of the electronic key being lost when dropped by the user.